This invention concerns a system and a method for the passive determination of the bearing of detectable objects and also the determination of the position of these by the monitoring of a large geographical area using a number of sub units.
Over a large geographical area--a border area, province, coastline, major road network, urban area, airport, etc--there is often a need to guard and monitor in order to be able to determine the position of objects that can be observed using physical measuring methods. The objects can for example be vehicles, boats, aeroplanes or helicopters and can for example be detected with the aid of the electromagnetic signals that the objects generate/transmit. Aeroplanes, helicopters and boats usually use some form of radar to assist navigation. A radar transmits electromagnetic signals within the radio frequency range. A radar's design varies for example depending upon the area of use, type of radar, frequency range used and upon whether it is airborne, or land- or sea-based.
In order to be able to determine the position of electromagnetic transmitting objects, such as electromagnetic emitters or transmitters within the radio frequency range, by cross-bearing, the bearing of the object must first be calculated from at least two different places. The calculation of bearings of electromagnetic transmitters can be carried out in many different ways of which interferometers, "Scan Time of Arrival" that measures the time of scanning in sub units, and "Differential Time of Arrival" (DTOA) methods are just some examples out of many.
For calculation of bearings by the interferometer method two receiver aerials in a sub unit should be placed at a distance from each other that is equal to one or several wavelengths of a target transmitter's carrier wave frequency. using two precisely electrically-matched receivers, one for each aerial, the difference in phase of the received carrier wave frequency between the two receivers is compared in an evaluation unit. The sub unit should be electrically symmetrical from the evaluation unit to both receiver aerials.
The phase difference at a particular frequency is a measurement of the time difference in reception by the respective receiver aerials that has arisen for the interesting signal due to the difference in path of propagation from the transmitter to the respective receiving aerials with full symmetry. A zero phase difference corresponds to equal paths of propagation, which means that the transmitter is located on the perpendicular from the midpoint of the line between the aerials (the base line for bearing measurement). The frequency of the carrier wave here consists of the relative time reference for the sub unit's two receivers. From the difference in the path of propagation the bearing of the transmitter relative to the direction of the base can be calculated with possible ambiguity if the phase difference exceeds 180.degree..
The measured bearing for an interesting signal is recorded together with a time specification for the measurement. The time specification should be sufficiently accurate for a corresponding measurement taken at another sub unit at a suitable distance from the first sub unit to be able with reasonable certainty to refer to the same transmitter situated in at least approximately the same place. Using reports from a number of sub units to a central unit an evaluation unit in the central unit can calculate the position of the transmitter. The evaluation unit uses bearings reported from at least two sub units situated at a suitable distance that were recorded at times that are so close that they can be taken as referring to one and the same scan. The length and geographical direction of the larger measuring base (the measuring base for cross-bearing) namely that between relevant sub units, must also be known.
The interferometer method can give a high degree of accuracy in bearings, but is relatively expensive and difficult to calibrate as a large number of well phase-calibrated receiver channels are required. The ambiguity if the phase difference exceeds 180.degree. must be resolved satisfactorily in order for the method to be able to give reliable bearings. The sensitivity is relatively low as a large signal/noise ratio is required in order to be able to measure the phase difference with a sufficiently high degree of accuracy. Also the data connection between sub units and information centre is complicated and expensive as the data rate must be sufficiently rapid in order to be able to transmit bearing data and times of bearing data with sufficiently high accuracy.
The method that is called DTOA is in principle similar to the interferometer method described above; the bearing of the transmitter can be determined using measured propagation time differences to two receiver aerials for signals from the same transmitter. Instead of measuring the phase difference, in this case the difference in time of arrival (TOA) is measured for both receivers of for example one and the same pulse.
Also here a symmetrical set-up is required and therefore any differences in propagation times in the connections should be compensated for in some way so that for the measured time difference zero between the received pulses in both receiver aerials, the transmitter is actually situated on the perpendicular through the midpoint of the base line. Methods of compensating for propagation time should fulfil the requirements for accuracy under varying external conditions, such as for example various ambient temperatures. The bearing measurements from a number of sub units are used otherwise in the same way as in the interferometer method to determine the position of the transmitter.
An accurate time measurement and a time synchronisation between sub units is necessary in this case. Also here complicated data connections and complicated sub units are usually required.
As mentioned "Scan Time of Arrival" is a third method of calculating bearings. The method is based on measuring the time of scanning of a radar scan with fixed rotational speed in different sub units located at relatively large distances from each other. A great disadvantage of Scan Time is that the method has a relatively narrow area of use.
An additional problem with these above-mentioned methods is the correlating of time measurements between different sub units and between sub units in pairs of sub units for the same transmitter. The correlation of bearings for the same transmitter between different sub units and between different pairs of sub units is also a problem. The transmitters must be classified in some way so that the times and bearings can be connected with the right transmitter.
The American patent U.S. Pat. No. 5,285,209 "Angle-of-Arrival Measurement via spectral estimation radar Time-of-Arrival periodicities" describes how DFT (Discrete Fourier Transformation) can be used on a series of TOA data to calculate "Pulse Repetition Frequencies" (PRF) for a pulse train or several superposed pulse trains. The American patent U.S. Pat. No. 5,396,250 describes in detail how the Fourier transformation is used to determine the properties of pulse trains with fixed or varying PRF. The method in accordance with U.S. Pat. No. 5,285,209 is used to determine the time difference for the reception of a pulse train between two receivers. The receivers work with a common reference clock. The bearing of a transmitter of the pulse train can be calculated from the difference in time/phase.
None of these patents deals with problems concerning the transmission of data between the sub units and a central unit, problems concerning the time synchronisation of individual receivers with the use of a large measuring base or problems concerning how a system for the determining of =position should be designed. In addition the use of calculation-intensive methods such as the Fourier transformation in sub units can be regarded as a disadvantage as this unavoidably leads to these units being complicated and expensive as the Fourier transformation requires a lot of resources as a result of being calculation-intensive.